A. Field of Invention
The present invention relates generally to sealing plugs and more particularly to a new and improved sealing plug for removably sealing access holes and other bores in metal castings, metal forgings and other metal parts, particularly those exposed to high pressure and/or vibration.
B. Description of Related Art
A variety of types of plugs have been designed for sealing hydraulic and other fluid systems, including simple expansion plugs as well as relatively complex plugs. High system pressures frequently require positive means of retaining the plugs, particularly when the opening to be plugged is large. A large opening results in an increased expelling force exerted on the plug, as a greater surface area is exposed to the system pressure. A commonly used design feature of plugs designed to seal large openings under high pressure is to mate male screw threads on the exterior of the plug with female screw threads in the interior of the installation site. The relatively high retentive force provided by the mated threads provides the ability to use large plugs that resist expulsion by high system pressures. A difficulty of the use of threaded plugs is that the plugs tend to loosen, allowing the plug to rotate and eventually back out, when the plug and/or the installation site are subjected to vibration. One method of design intended to prevent the unscrewing of threaded plugs includes using glues, plastics or other binding agents on the threads. Using permanent glues hinders removal and other types of binding agents that allow removal frequently are not reusable or have limited reusability. Another design used to prevent unscrewing is the locking ring type plug, similar in principle to the fitting shown in U.S. Pat. No. 3,702,707 to Rosan. Rosan describes the use of a ring that slides over, but is rotationally fixed relative to the fitting body. The Rosan ring has a serrated outer surface and inner surfaces with outer splines that are inserted into matching serrations in the boss of the installation site and inner splines that are to be inserted into matching serrations in the fitting body. The body also has screw threads that seal and secure the body within the opening to be plugged. The inner ring splines prevent rotation of the body relative to the locking ring and the outer ring splines prevent the ring from rotating in the installation site. A disadvantage of the locking ring type plug is the requirement of an adaption of the opening by the relatively complicated and time consuming formation of the serrations at the mouth of the installation boss which must be relatively large as a result. Since the locking ring is driven into the installation opening, the ring itself may back out of locking position and therefore a means of locking the locking ring is required. In addition, the installation of a locking ring type of plug requires that the plug body serrations, and the installation boss serrations must exactly line up to engage the ring splines. For this reason it is necessary to adjust the rotational position of the plug until a proper alignment is achieved, a somewhat time consuming and delicate process. A further disadvantage of the locking ring type design is that the necessity of locking the ring in place makes it difficult to make a plug that is flush or nearly flush with the surface of the installation site. Plugs of this type also frequently employ elastomeric seals that have limited useful lives and limited usefulness in some temperature conditions or applications involving incompatible substances.
Therefore, what is desired is a plug that is reusable, economic to manufacture, easy to install, useful in a wide range of conditions and that resists the influence of extraneous vibration and can be configured to be flush with the surface of the installation site when installed, without unnecessary weight.